Not sure if you're into "browsergames" , but I've tried a few (magewar, dragon tavern, travian and some others) and this BvS has managed to stand out of the lot.
The nature of the game is fairly straightforward, but once you're playing for a few days you quickly see that the game is VERY LARGE, and has many features to keep a casual interest.
You're a ninja, an anime ninja to be exact.
Thus, you use your skills with a touch of comedy and flair. Hang out with friends, spar, manage a village, fight monsters and more.
There's a lot to the game, and they have a forum as well in case you got questions or get stuck on a mission.
Anyway, If you're not shy to try a new thing, have a look here.